docs: def merge(old_df, new_df, return_index=False):
          """
          Merge two dataframes of buildings.  The old dataframe is
          usually the buildings dataset and the new dataframe is a modified
          (by the user) version of what is returned by the pick method.
  
          Parameters
          ----------
          old_df : dataframe
              Current set of buildings
          new_df : dataframe
              New buildings to add, usually comes from this module
          return_index : bool
              If return_index is true, this method will return the new
              index of new_df (which changes in order to create a unique
              index after the merge)
  
          Returns
          -------
          df : dataframe
              Combined DataFrame of buildings, makes sure indexes don't overlap
          index : pd.Index
              If and only if return_index is True, return the new index for the
              new_df dataframe (which changes in order to create a unique index
              after the merge)
          """
          maxind = np.max(old_df.index.values)
          new_df = new_df.reset_index(drop=True)
          new_df.index = new_df.index + maxind + 1
          concat_df = pd.concat([old_df, new_df], verify_integrity=True)
          concat_df.index.name = 'building_id'
  
          if return_index:
              return concat_df, new_df.index
  
          return concat_df